In general, blood purification treatment such as dialysis treatment, a blood circuit made of flexible tubing is used to place the blood of patient in extracorporeal circulation. This blood circuit arrangement mainly includes an arterial blood circuit having an arterial needle at one end to collect blood from the patient and a venous blood circuit having a venous needle at the other end to return the blood to the patient. A dialyzer between the arterial blood circuit and the venous blood circuit purifies the blood circulating extracorporeally.
Such dialyzer is made of plural hollow fibers inside the dialyzer. Blood flow inside each hollow fiber and dialysate flowing outside the hollow fibers (i.e. between external surface face of the hollow fiber and inside surface face of the dialyzer case). The hollow fiber is a blood purification membrane with small pores on its surface. Wastes in the blood flowing inside the hollow fiber are discharged into the dialysate after passing through the blood purification membrane, and the purified blood returns to the patient.
Meanwhile, the pressure of the patient's blood flowing in the venous blood circuit is monitored during dialysis treatment, and when the pressure is remarkably increased or decreased, it is determined that an abnormality has occurred and that the abnormality is reported to a medical worker by an alarm. Specifically, a pressure sensor connected to a monitoring tube extending from the air-layer side of a venous drip chamber is attached to the venous blood circuit and is used to constantly monitor the blood pressure of the patient's blood flowing in the venous blood circuit. If the measured pressure by the pressure sensor is out of a predetermined range, which is an upper and lower values of an alarm-threshold, the event is alarmed as a possible occurrence of abnormality.
In such a blood purification device, the alarm threshold set at the beginning of the treatment is always constant during the treatment, and when it is reset, the threshold becomes constant. However, there are following issues remained to be solved.
Normally, the venous blood pressure, as shown in FIG. 6, varies depending on the patient's condition during the dialysis treatment. The alarm-threshold, upper value D1 and lower value D2, are set as in FIG. 6 until reset and when the pressure is beyond upper value D1 at T1, an alarm would be activated even though no abnormality has occurred.
When the patient changes sides or position, or a substitution fluid is added to the patient during medical treatment, the venous pressure measured varies and according to such change, an alarm sometimes is activated because the pressure is beyond upper value D1 or lower value D2. Such frequent false alarms produces undue burden on a medical worker.
Upper value D1 and/or lower value D2 are set larger in some medical agency to avoid activating false alarms, so that when an event of abnormality on extracorporeal circuit occurs because of, for example, changes in patient's condition or clogging of the dialyzer, or the needle coming off from the patient or the needle separating from the blood circuit, the changes of venous pressure, which must be unavoidably alarmed, will not be detected. For example, even when the needle comes off from the patient or the needle separates from the blood circuit and the blood pressure is lower than the dotted line in the same figure, the pressure measured does not go beyond lower value D2, and as a result, no alarm is activated.
When the needle comes off, in many cases, the decrease of venous blood pressure is very small even though the flow resistance on introduction of the blood to the patient body is null because some flow resistance is generated by the needle. Such a small decrease can be hardly detected. Therefore a false alarm is frequently activated, making the detection of actual abnormality difficult, the abnormality being related to sudden changes in the patient, the needle coming off from the patient, the needle separating from the blood circuit, or defects of the extracorporeal system.